Photoaging of human skin is a complex response due to inflammation, oxidative injury, cellular and extracellular changes induced by decades of sunlight exposure. UV wavelengths are thought to be mainly responsible. Both of the primary skin layers, epidermis and dermis, are affected. Epidermal photoaging includes pigmentary lesions called ephilides (freckles) and solar lentigines (larger pigmented spots), plus pre-cancerous clonal lesions of keratinocytes called actinic keratoses. Thermal destruction of part or all of the epidermis, the outermost cellular layer of skin about 0.1 mm thick, is an effective treatment for epidermal photoaging. For example, lasers that vaporize epidermis are highly effective in a treatment called laser resurfacing. However laser resurfacing creates a significant skin wound with risk of infection, and prolonged healing. Dermal changes of photoaging include solar elastosis (an accumulation of abnormally-formed elastin fibers in the upper reticular layer of the dermis), laxity, loss of elasticity, fine and coarse wrinkles. Laser resurfacing to a depth below the dermo-epidermal junction can be highly effective for improving dermal photoaging, through a process of stimulated wound healing. Deep chemical peels, dermabrasion and other methods of destruction of epidermis and/or dermis are also effective, and also produce a significant open skin wound with risk of infection and delayed healing.
Patterns of stimulated thermal damage to epidermis and/or dermis are also effective for treatment of photoaging. Recently, “fractional photothermolysis” using mid-infrared lasers to produce a microscopic array of thermal injury zones that include both epidermis and dermis was reported to be effective and well-tolerated for treatment of photoaging (D. Manstein et al. “Fractional Photothermolysis: a new concept for cutaneous remodeling using microscopic patterns of thermal injury.” Lasers Surg Med 34:426-438, 2004). A primary advantage of fractional photothermolysis is that each zone of thermal injury is smaller than can be easily seen with the unaided eye, and surrounded by a zone of healthy tissue that initiates a rapid healing response. As described Manstein, the epidermis is stimulated to heal rapidly and without creating an open wound. The microscopic zones of thermally injured epidermis slough harmlessly from the skin surface after several days to several weeks, leaving a rejuvenated epidermis with less photoaging changes. Repeat treatments, which are well tolerated, can be performed until a desired result is obtained. The microscopic zones of thermal injury with fractional photothermolysis extend well into the dermis, as well. Dermis does not heal as rapidly as epidermis, in general. Over weeks to months following treatment, some of the abnormal dermis due to photoaging is remodeled, however, leading to improvement in laxity, wrinkles and skin texture.
Fractional photothermolysis (FP) is intrinsically limited to regions of approximately the upper 1-millimeter of skin. The basic concept of producing well-controlled arrays of thermal injury is therefore limited with fractional photothermolysis, to superficial aspects of photoaging. Aging, which also causes laxity of the skin, and photoaging involve deeper layers of the dermis. Solar elastosis can extend throughout the dermis, to approximately 3 mm deep or more. Laxity and loss of elasticity due to aging are bulk problems of the dermis.
A fundamental requirement for producing arrays of small thermal injury zones using a source of radiant energy that propagates and is absorbed within tissue, is that the source of radiant energy be capable of being adequately delivered to the tissue depth for which the array is desired. Near the skin surface, light can be used, as in fractional photothermolysis. However, light that propagates more than about 1 mm through skin has been multiplied scattered, and can no longer be focused or delivered.